Start with the job the battery must do
There is no single battery size that fits every commercial site. A system intended to keep essential equipment running during an outage is sized differently from one intended to reduce short demand peaks or store daytime solar generation for later use. Before comparing products, decide whether the main goal is backup power, peak shaving, solar self-consumption, or a combination of these.
For backup, define which loads must stay on. Supporting a critical-load circuit can require far less energy than supporting an entire factory or building. For peak shaving, the important questions are how high the unwanted peaks are, how long they last, and how often they occur. For solar self-consumption, the timing of PV production and site demand matters as much as the daily energy total.
kW and kWh answer different questions
Power (kW) is the rate at which the system can deliver electricity at a given moment. It must be sufficient for the loads that need to run together. Motors and other equipment may also have starting demands above their normal operating power.
Energy (kWh) is the amount of electricity available over time. A simplified duration estimate is energy divided by average power. For example, 100 kWh divided by a steady 50 kW load equals two hours before accounting for operating limits and losses. A battery described only by its kWh rating therefore does not tell you whether its inverter can support the required kW load.
An illustrative backup calculation
Suppose a site identifies 50 kW of critical loads and wants them supported for two hours. The simple starting calculation is:
50 kW × 2 hours = 100 kWh delivered to those loads.
This is a planning example, not a recommended equipment size or a performance guarantee. The battery’s nameplate capacity may need to be higher than 100 kWh because not all stored energy is usable and the conversion path has losses. The power-conversion equipment must also meet the simultaneous load and any relevant starting demand. If the load changes during the two hours, use a time-based load profile rather than assuming a constant 50 kW.
The same 100 kWh figure cannot be used unchanged for every application. Peak shaving depends on the shape and frequency of demand peaks; solar self-consumption depends on surplus PV generation and later consumption; backup depends on the critical loads and the outage duration to be covered. A system serving more than one purpose needs an operating strategy that reserves enough energy for the most important requirement.
What changes the final system size?
- Load profile: Interval data reveals peak power, daily patterns, and whether the assumed average load is realistic.
- Usable battery capacity: Operating limits and reserve settings affect how much of the nameplate energy can be delivered.
- Conversion and system losses: Inverter and other electrical losses reduce the energy available at the load.
- Starting and peak loads: Equipment that starts suddenly can require more power than its steady-state rating suggests.
- Solar and grid configuration: PV output, grid availability, charging opportunities, and operating mode affect the required battery reserve.
- Site and local requirements: Space, environment, interconnection, fire-safety provisions, and local rules must be reviewed for the actual installation.
These factors should be checked together. Choosing a battery by kWh alone, or choosing an inverter by kW alone, can leave an otherwise plausible design unable to meet the site’s operating goal.
Information to send when requesting a proposal
To move from a rough estimate to a project-specific design, provide the site location and intended application; recent interval load data or utility records; a list of critical loads and any large motors; the target backup time or demand-reduction goal; existing or planned PV capacity; grid and generator details; available installation space; and any known local technical or permitting requirements. If some information is not yet available, identify the gaps so they can be checked during design.
Explore NOVASUN’s commercial and industrial energy-system approach and solar-system range. When you are ready to discuss your site, contact NOVASUN with the information above for a project-specific assessment.